Field experience (FE) is a key component of pre-service English language teacher education enabling the early integration of pre-service teachers (PSTs) in real teaching situations. As a well-researched area, FE is known to increase PSTs' teaching-efficacy perceptions. Thus, to better understand how it does so in the Turkish context, this study examines senior PSTs' (N=145) teaching-efficacy perceptions and the potential sources of these perceptions in the course of FE. Adopting a pretest-posttest design supported by semi-structured interviews (N=30), the study showed a significant increase in the PSTs' teaching-efficacy perceptions from pre-test to post-test (p<0.05). While the interview data indicated that "prior teaching experience, decreased sense of teaching anxiety, and increased confidence in professional self" were among the major sources of the increase in these perceptions, such sources as "untested teaching competencies" and "critical awareness of teaching and profession" had a negative impact on the PSTs' efficacy perceptions.